Lore

Clerics transformed into the most hideous beasts.

The Cleric Beast, like those that came before it, is a Cleric of the Healing Church who has given over to beasthood completely. Ludwig was the first of many Healing Church hunters to come, many of whom were clerics. As it was, clerics transformed into the most hideous beasts.

Seeking a way to become more than human the scholars of Byrgenwerth discovered old blood within the chalice dungeons below Yharnam. The discovery of blood made their dream of evolution a reality. Metamorphosis, and the excesses and deviation that followed, were only the beginning, however, the discovery of blood also entailed the discovery of undesirable beasts. Because of this the discoveries related to beasthood were deemed forbidden, as seen in the description for the Beast Caryll rune, and the discoveries were likely forbidden by Willem. Many of the students at Byrgenwerth however, did not agree with Willem in his views on blood and wished to continue studying its effects. These events led to the schism at Byrgenwerth where many of the students departed the institution, including Laurence, who would go on to found the Healing Church.

Members of the Healing Church continued to study the old blood, and would partake of it in a ritual called Communion. Vicar Amelia is found repeating a mantra that seemingly represents the teachings of the Healing Church members:

Seek the old blood.
Let us pray… to partake in communion.
Let us partake in communion… and feast upon the old blood.
Our thirst for blood satiates us, soothes our fears.
Seek the old blood… but beware the frailty of men.
Their wills are week, minds young.
The foul beasts will dangle nectar and lure the meek into the depths.
Remain wary of the frailty of men.
Their wills are weak, minds young.
Were it not for fear, death would go unlamented.

Amelia's mantra illuminates a number of key points. Members of the Healing Church would feast upon the old blood. Drinking the blood satiated a need or desire within its members, driving away their fears. But there was always an inherent danger in partaking of the old blood. Members understood that mankind was frail, that their willpower is weak and their minds young, especially in comparison to more advanced beings like the Great Ones. When partaking in blood, there was always the temptation to embrace beasthood. The Beast Roar description indicates that beasthood lurks deep within the frames of men, a nectar luring the meek into the depths. This beastly nature offers strength, relief from all fear, but it comes at a high cost.

Laurence, first vicar of the Healing Church became the first cleric beast. And it seemingly was not long before others joined him. Brador, a Church Assassin and hunter killed his compatriot after they transformed into a Cleric Beast, skinning the beast thereafter and wearing its hide as testimony of what he had seen and done. He brought this testimony to the Healing Church, who swore him to secrecy, providing him with a soundless bell of death that would allow him to keep their secrets.

But this secret, like the beasthood within mankind, could not be contained so easily. When clerics began transforming into unspeakable beasts, the Church needed something to retaliate with. Weaponry and techniques were developed to deal with Cleric Beasts. One example of this is the golden radial blood gem, kept for generations within the Church that strengthens attacks against those afflicted intensely by beasthood.

Perhaps across these generations the Church searched for a cure to the beasthood that plagued them, but like Brador's compatriot, our Cleric Beast serves as testimony to the Church's failure.

Theories

Beasthood is indelibly linked to willpower.

From Drained of Blood interview with Mr Miyazaki: The urge to transform into a beast is in conflict with the basic sense of humanity we all have. That humanity serves as a kind of shackle, keeping the transformation in its place. The stronger the shackle keeping that urge to transform in place is, the larger the recoil once that shackle is finally broken. The results cause you to transform into a larger creature, or a more twisted one. The struggle between these two urges is one concept here. You see that pretty clearly with the beast characters designed early on – especially the Cleric Beast, which serves as their icon of sorts. That connects with the idea that the cleric is really the most fearsome beast of them all.

Sources

Sword Hunter Badge
One of the badges crafted by the Healing Church. The silver sword is a symbol of a Church hunter
Ludwig was the first of many Healing Church hunters to come, many of whom were clerics.
As it was, clerics transformed into the most hideous beasts.

Clockwise Metamorphosis Rune
A secret symbol left by Caryll, runesmith of Byrgenwerth.
The twisted cross means "metamorphosis."
Rotated clockwise, this rune boosts HP.
The discovery of blood made their dream of evolution
a reality. Metamorphosis, and the excesses and deviation
that followed, were only the beginning.

Beast Rune
A secret symbol left by Caryll, runesmith of Byrgenwerth.
A transcription of the roar of a labyrinth beast, the bearer of
the "Beast" rune has accentuated temporary transformation
effects.
"Beast" is one of the early Caryll Runes, as well as one of the
first deemed forbidden. The discovery of blood entailed
the discovery of undesirable beasts.

Gold Blood Gem
A blood gem that fortifies weapons and adds various
properties.
Most radial blood gems have effects that bear upon physical
attacks, and this golden radial blood gem, kept for
generations within the Church, strengthens attacks against
those afflicted intensely by beasthood.
When clerics began transforming into unspeakable beasts,
the Church needed something to retaliate with.

Laurence's Skull
Skull of Laurence, first vicar of the Healing Church.
In reality he became the first cleric beast, and his human
skull only exists within the Nightmare.
the skull is a symbol of Laurence's past, and what he failed
to protect. He is destined to seek his skull, but even if he
found it, it could never restore his memories.

Brador's Testimony
The scalp of a horrid Cleric Beast, indicating that hunter Brador,
a Healing Church assassin, had killed a compatriot.
Afterward, he wore his ally's own scalp, and hid himself away, deep below in a cell.
The Church provided him with a single, soundless bell of death to ensure their secrets would be kept.

Beast Hide Garb
The bloodied hide of a horrible Cleric Beast, pulled over the back.
Without the attached beast hide this foreigner garb wouldn't raise anyone's eyebrows.
Brador donned a compatriot's beastly scalp and hide while still moist with blood.
Most of the blood stains on this hide were from that day

Vicar Amelia dialogue:
"Seek the old blood.
Let us pray… to partake in communion.
Let us partake in communion… and feast upon the old blood.
Our thirst for blood satiates us, soothes our fears.
Seek the old blood… but beware the frailty of men.
Their wills are weak, minds young.
The foul beasts will dangle nectar and lure the meek into the depths.
Remain wary of the frailty of men.
Their wills are weak, minds young.
Were it not for fear, death would go unlamented."

Communion Rune
A secret symbol left by Caryll, runesmith of Byrgenwerth.
Several runes relate to "Blood," including "Communion,"
which raises the maximum number of blood vials
one may carry.
The rune represents the Healing Church and its
ministers. Blood ministration is, of course, the pursuit
of communion.